Combustion processes may be used in power plants to generate heat for operating a steam generator, which generates electric power. The fuel used for such processes may include coal, petroleum coke, and/or biofuel derived from biomass. The fuel may include an alkali-containing material. Other alkali-containing materials known to those skilled in the art may be used in the processes to, for example, capture environmental pollutants.
Some power plants may include systems that operate using, for example, a process sometimes referred to as a “fluidized-bed combustion” process. One example of such a process is a circulating fluidized-bed combustion process, which may be used for electric power generation. Some examples of circulating fluidized-bed reactors may include gasifiers, combustors, and steam generators, and typically, circulating fluidized-bed reactors have an upright furnace or boiler.
During operation, fuel, for example, particulate fuel, is introduced into a lower part of a furnace, and primary and secondary gases, for example, air, may be supplied through a bottom and/or sidewalls of the furnace. Combustion of the fuel takes place in a bed of fuel particles and other solid particles, such as, for example, calcium carbonate, which may be included for sulfur dioxide capture, and/or inert material. For example, the fluidized-bed reactor (i.e., furnace) may be configured to suspend the bed of fuel particles and other materials on upward-blowing jets of the primary and/or secondary gases during the combustion process. The upward-blowing jets facilitate mixing of the fluid particles and other materials, which serves to improve combustion by, for example, reducing undesirable emissions and increasing combustion and heat transfer efficiency.
Exhaust gas and/or solid particles entrained in the bed may leave the furnace via an exhaust port in, for example, an upper part of the furnace and may be passed to a particle separator. In the particle separator, most or substantially all of the solid particles may be separated from the exhaust gas. Typically, one or more cyclones, which use tangential forces to separate particles from exhaust gas, are coupled with the furnace. During normal operation, cyclones may be capable of separating about 99.9% of the particles from the exhaust gas.
The exhaust gas and any remaining solid particles, or fly ash, may then be passed through additional processing units before ultimately being released into the atmosphere. For example, in an atmospheric circulating fluidized-bed system, the exhaust gas flows through a boiler and past its boiler tubes containing a supply of water, providing heat to convert the water to steam. The steam may then be used to drive a steam turbine, generating electricity. The exhaust gas may be passed through a heat exchanger to recover at least a portion of the heat generated during the combustion process, and the exhaust gas may be passed through environmental processing units to reduce levels of undesirable emissions, such as pollutants, for example, nitrogen oxides (“NOx”), sulfur oxides (“SOx”), and/or particulate matter (“PM”).
Solid particles recovered in the particle separator, such as bottom ash, may be returned to the bed in the circulating fluidized-bed reactor for subsequent reaction and/or removal from the bed. Energy bound in the heated bottom ash may be at least partially recovered, for example, in an integrated fluidized-bed heat exchanger, before the ash is recycled to the circulating fluidized-bed reactor.
An exemplary integrated fluidized-bed heat exchanger is an INTREX™ steam superheater (Foster Wheeler Ltd.; Clinton, N.J., USA). In such a heat exchanger, bottom ash separated in a cyclone may pass over the INTREX™ steam superheater before returning to the circulating fluidized-bed reactor. The use of other fluidized-bed heat exchangers known to those skilled in the art is contemplated.
Combustion of the fuel particles and/or heating of other materials (e.g., calcium carbonate) may result in heating of alkali-containing materials, such that alkali compounds contained therein are released. The released alkali compounds may react with ash or other inorganic components, such as, for example, sulfur, chlorine, and/or silicon, which may result in undesirable deposits, ash accumulation, and/or corrosion occurring on exposed surface areas of the fluidized-bed components, for example, on furnace walls and/or boiler tubes. Such deposits and corrosion may lead to less efficient operation and/or lost production due to increased maintenance-related down time. Without being limited by theory, the alkali compounds may be released in a liquid or vapor form, which may be entrained in the fluidized bed or with the particles making up the fluidized bed. The alkali compounds may cause ash particles to stick together, leading to an undesirable ash accumulation (e.g., on boiler tubes) and fouling of the reactor system surfaces. Without being limited by theory, the alkali components and siliceous component of the ash may form a eutectic mixture that form crystalline/amorphous deposits on the reactor surfaces.
As a result, it may be desirable to remove at least a portion of the alkali compounds from the furnace before they react with the ash and/or other inorganic components, for example, to reduce or prevent undesirable deposits and/or corrosion.
Davidsson et al., in an article entitled, “Kaolin Addition during Biomass Combustion in a 35 MW Circulating Fluidized Bed-Boiler”, Energy & Fuels 2007, 21, 1959-1966, describe adding kaolin to a circulating fluidized-bed boiler. Davidsson et al. specify using kaolin sold under a product name Intrafil C® and obtained from Imerys Minerals Ltd., and state that the addition of this highly-processed kaolin results in removal of alkali from the furnace. In particular, the kaolin used by Davidsson et al. is highly processed and may have a very low moisture and/or iron content.
Although the kaolin added by Davidsson et al. to the circulating fluidized-bed boiler may result in the removal of alkali from the furnace, the method described by Davidsson et al. may suffer from a number of possible drawbacks. For example, the kaolin added is a highly processed, fine powder kaolin, and an undesirably large portion of the kaolin was carried out of the circulating fluidized-bed reactor by the flue gases, and thus only a relatively small fraction of the kaolin remained in the furnace. This may result from, for example, the fineness of the kaolin, 36% of the kaolin having a particle size distribution of less than 1 μm, and 55% having a particle size distribution of less than 2 μm. Davidsson et al. indicate that an undesirably high amount of the kaolin ended up in the fly ash. Moreover, the kaolin used by Davidsson et al. may not be sufficiently cost effective due to the costs sometimes associated with such highly processed kaolin.
In light of these possible drawbacks, it may be desirable to identify a less costly method for removing alkali from a furnace, for example, the furnace of a fluidized-bed reactor.